Telomeres are protective sequences of DNA that sit at the tips of our chromosomes and have been compared to the caps at the end of shoestrings. They get shorter with age. Cells with critically short telomeres can no longer divide, making short telomeres a hallmark of aging. Elizabeth Blackburn made the discovery of telomeres in 1978, and in 1985 she discovered the enzyme telomerase, which can lengthen telomeres. She won the Nobel Prize for this work, which began in single-celled organisms called Tetrahymena thermophila, commonly known as pond scum.